Acoustic horn assembly



W. E. GLENN Oct. 9, 1956 ACOUSTIC HORN ASSEMBLY 2 Sheets-Sheet 1 FiledMarch 14, 1955 SOURCE FREQUENCY I'A/ CYCL ES/JE'COA/D FREQUENCY l/YCYCLES /SC0IV0 n9 m? ZEWM w v w 1W 2 Sheets-Sheet 2 O t. 9 1956 W. E.GLENN ACOUSTIC HORN ASSEMBLY Filed March 14, 1955 {MM/q n w 6 r m m 1 51FREQUENCY l/V CYCLE-S /SCOA/0 ACOUSTIC HORN ASSEMBLY William E. Glenn,Schenectady, N. Y., assignor to General Electric Company, a corporationof New York Application March 14, 1955, Serial No. 494,159

7 Claims. (Cl. 181-31) This invention relates to acoustic hornassemblies which are suited for utilization in combination with anacoustic diaphragm such as that commonly utilized for a loud speaker.While this invention is subject to a large number of modifications andvariations, it is ideally suited for utilization with a conventionalloud speaker and is particularly described in this connection.

It is known that the response of an acoustic diaphragm and/or loudspeaker assembly is a function not only of the speaker structure itselfbut also of the accompanying speaker enclosure. In order to obtainfaithful reproduction of low frequency sound waves, the sound wavesformed on the back side of the acoustic diaphragm of the speaker ortransducer may be passed through a horn of increasing cross-sectionalarea. Such horns are satisfactory for high fidelity sound reproductionbut best results are achieved only with a horn of such length as to bevery cumbersome even though the horn is composed of labyrinthconvolutions within the speaker enclosure or cabinet.

An important characteristic of horns is that their transformer-likeproperty improves the impedance match between the speaker and air,particularly at low frequencies. This results in a significant reductionin the speaker excursion or motion for a given sound energy output and,consequently less distortion.

At low frequencies, the size of the mouth of the horn is necessarilyrelatively short compared to a wave length. Consequently, the acousticalimpedance of the air load does not match the impedance of the mouth ofthe horn. A condition then results which is somewhat analogous to thatof a transmission line which is not terminated in its characteristicimpedance. Since, under these conditions, the energy is not all absorbedat the mouth, some of it reflects back down the horn and tends to eitherreinforce or cancel the original signal. This results in largefluctuations, with changing frequency, in the impedance at the throat ofthe horn and a resulting nonuniform frequency response. In addition,these fluctuations in horn throat impedance result in a high excursionof the speaker diaphragm at certain frequencies and distortion.

It is therefore an important object of this invention to provide anacoustically loaded horn assembly having a substantially flat frequencyresponse with any desired degree of sharp cutoff.

Another object of this invention is to provide an improved acousticalhorn.

An additional object of this invention is to provide an acoustical hornassembly which for a given level of sound quality occupies a smallerspace for a given frequency response and distortion than do horn-typeassemblies previously known.

It is also an object of this invention to provide a combinationacoustical horn and acoustical diaphragm assembly.

An additional object'of this invention is toprovide an States Patent'ice acoustical horn assembly which is effectively terminated in itscharacteristic impedance.

In accordance with an aspect of this invention there is provided animproved acoustic horn assembly in which an acoustical horn has anincreasing cross-sectional area from the throat of the horn to the mouthof the horn. Between the throat and mouth of the horn acoustic loadingmeans are interposed to obtain a desired frequency response which mayinclude a substantially flat low distortion output with a sharp cutoff.In a specific embodiment the horn is effectively terminated in itscharacteristic acoustical impedance as seen from the throat of the horn.

In an exemplary embodiment of this invention the throat of the horn iscoupled to the back radiating side of an acoustic diaphragm and themouth of the horn is oriented in proximity to the front radiating faceof the acoustic diaphragm so that the high frequency sound componentsare radiated by the front side of the acoustic diaphragm and the lowfrequency components are radiated by the horn with substantially noreflections from the mouth of the horn.

Other important objects and aspects of this invention will becomeapparent from the following description particularly when taken with theannexed drawing in which Figures 1 and 2 illustrate a side elevation andtop view of a preferred embodiment of this invention; Figures 3, 4 and 5illustrate characteristic response curves useful in explaining operatingcharacteristics obtainable with the practice of this invention andFigure 6 illustrates an alternative embodiment of this invention.

In accordance with the practice of this invention an acoustical horn isloaded with acoustical impedance material so that the horn appears fromthe throat end to be terminated in its characteristic impedance. Thus,the impedance loading of the acoustical horn is essentially resistiveand of a magnitude substantially equal to the characteristic impedanceof the horn.

It may be shown that a sound radiator that is small compared to a wavelength has a relatively fiat frequency response with a substantiallyconstant driving force if it is mass loaded. It is also known that ifthe radiator is resistance loaded the output tends to increaseapproximately 6 decibels per octave as the frequency is increased.Therefore, a terminated horn with a mouth which is small compared to awave length will have an output which increases approximately sixdecibels per octave as the frequency increases.

In order to compensate for the tendency of the horn to emphasize thehigh frequency audio components more than the low frequency componentsadditional electrical or mechanical compensation must be provided.Mechanical attenuation can take the form of masses of sound absorbingmaterial located in at least one region throughout the length of thehorn so as to provide the necessary high frequency compensation toresult in substantially flat horn output. This form of mechanicalcompensation is more satisfactory than the electrical compensation,since it avoids the necessity of utilizing complicated circuitry, bystufling portions of the horn with sound absorbent material. Inaddition, this form of acoustical loading decreases the eifect ofharmonic distortion occurring at the throat of the horn.

Applying these fundamental relations to a specific example, it may beassumed that there is provided a loud speaker with a horn coupled to theback radiating surface of the speaker. The mouth of the horn is locatedin the vicinity of the front radiating face of the speaker. In order toprevent impedance fluctuations throughout the frequency response rangeof the speaker it is necessary to terminate the horn resistively in thecharacteristic impedance of the horn.

Therefore, a way of making up the natural tendency of the horn toincrease the amplification of high frequency .components approximately 6decibels per octave is to design the horn so that the sound output ofthe horn at cutoff approximately matches the front radiation from thespeaker at high frequencies and to distribute the loading material inthe horn so that it will attenuate the high frequency componentsapproximately 6 decibels per octave.

In accordance with a preferred embodiment of this invention there isprovided the structure illustrated in Figures 1 and 2 wherein speakercabinet assembly lt'i, which is adapted to be placed in the corner of aroom, includes top member 11 and bottom member 1'2 with side panels 13and Old. The enclosure is completed by vertical strips 15 and '16 whichare rigidly secured together by panel 17. Supported from the side panelsand the top panel is baffle 18 which is rigidly secured to front panel17 by baffle member 19. Opening 2th is provided in bafle l8 and acousticdiaphragm 2-1 of electroacoustic transducer assembly 22, for example aloud speaker, is mounted across this hole. A source of audio signals 23is coupled by lead 24 to the speaker 22. In accordance with the practiceof this invention sound absorbing material 25 terminates the end of thehorn and provides distributed impedance throughout the length of thehorn. In addition, a second region 26 of sound absorbing material isprovided in the horn.

In accordance with the practice of this invention high frequencycomponents are radiated by the front surface of acoustical diaphragm 21and out through opening 27. Low frequency components from the bacxsurface of the acoustical diaphragm, indicated by the arrows L, passthrough the horn of expanding cross section and emerge through opening27. The masses of sound absorbing material 25 and 26 effectivelyattenuate the high frequency components of the audio input approximately6 decibels per octave and the mass of sound absorbing material 25 alsoprovides a resistive termination for the horn approximately equal to thecharacteristic impedance of the mouth of the horn. The mass of soundabsorbing material 25 replaces the mass of air of an equivalent longerhorn and results in a considerable volume saving since the mouth end ofthe horn occupies the larger volume.

With loading material distributed in the horn, such as that in Figures 1and 2, it is apparent that the horn passes frequencies at which the pathlength through the By way of example, design criteria, which may beutilized in the practice of this invention, appear in the succeedingparagraphs and characteristic response curves obtained with the practiceof this invention appear in Figures 3 to 5.

The excursion of the acoustical diaphgram for a given sound input isreduced by the use of a horn. It can be shown that this reduction isapproximately equal to the ratio of the horn mouth area to the hornthroat area and that to obtain significant improvements by the use ofhorn rather than a reflex enclosure, this ratio should be of the orderof or less than one to six.

The throat area should be chosen so that the mechanical impedance of thehorn is lower than the mechanical impedance of the speaker at the lowercutoff frequency of the system. For impedance match, it may be shownthat the horn throat area A in square feet is approximately defined by:

where Cu is the suspension compliance of the acoustical diaphragm incentimeters .per dyne, fc is the low frequency cutoff of the horn and Ris the effective speaker in inches. For example, if the speaker has amechanical compliance of l0-' centimeters per dyne, a typical value,then the throat area in square feet is:

The throat area should, therefore, be this value or smaller.

The horn mouth area is determined by the necessity of matching theoutput of the horn at cutoff to the high frequency output of thespeaker. This condition is satisfied if:

Am/A.=(fr/fc) where Am is the horn mouth area, A is the throat area, fris the speaker resonant frequency and fo is the effective low frequencycutoff of the horn.

The effective horn cutoff frequency for a terminated horn with a mouthto throat area ratio of 6 to l is about 1.6 times the cutoff frequencyof an infinite horn. Taking this approximation into consideration, thedistance in which the horn must flare a factor of two in area can begiven.

Using the above criteria some typical design characteristics of horns inaccordance with this invention appear in Table I.

Table I Speaker Approx. Speaker Resonance A in sq. Mouth Throat HornHorn Vol. Acoustic Diameter Frequency, ft. from Area Am Area At Cutoff.in cu. ft. Power at Cycles Equation 1 in sq. ft. sq. ft. 0. P. S. 40 C.I. S. per sec.

horn in a wave length or more. It will be apparent that this results inserious fluctuations in the frequency response at high frequencies dueto cancellation or addition of the front radiation of the speaker 21 andthe sound coming through the horn. Consequently, it is generallynecessary to place padding material in at :least two regions along thehorn so that the horn acts effectively as a sharp cutoff acoustic filterwith a high frequency cutoff which is just below the frequency at whichthe length of the horn is one wave length.

In this manner, faithful reproduction of the high frequency componentsof the audio input and of the low It will be noted that the volume givenis approximately the minimum volume for an ideal exponential horn. Itwill be appreciated that the actual volume will be somewhat larger thanthis because of the volume occupied by the materials utilized to formthe speaker enclosure and the waste space involved in folding anexpanding horn into a reasonably shaped enclosure.

It will be noted that Table I lists the approximate acoutic power inwatts that will be available at 40 cycles per second from typicalcommercially available speakers in the enclosures indicated by thetable. This is about 36 times the power available from the same speakerwhen frequency components of the audio input is obtained. mounted in aninfinite battle.

It will be noted that for large speakers the volume necessary to havethe low frequency match the high frequency components becomes excessive.In these cases, a practical alternative solution may be to reduce thearea of the horn. This reduces the low frequency output and the speakerexcursion. The low frequency components can then be boostedelectrically. Since the speaker excursion for a given sound outputdepends on the mouth to throat area ratio of the horn, there will stillbe a reduction in the inherent distortion.

Figure 3 shows the mechanical impedance (force-current analogue) of thethroat of a horn with and without acoustical termination as a functionof frequency. Curve 28 is that of an unterminated horn and dashed linecurve 29 is that of a horn terminated in accordance with this invention.The points of high impedance correspond to frequencies at which thespeaker excursion is high for a given sound output. For example,impedance measurements of this sort can be obtained by constructing abridge circuit that balances out the electrical impedance of theelectro-mechanical transducer associated with the acoustical diaphragmso that only the mechanical impedance is measured.

The resulting frequency response is shown in Figure 4 wherein curve 30is representative of the frequency response of a terminated horn anddashed line curve 31 of an enclosure such as that illustrated in Figures1 and 2 without the termination. It is apparent from this curve that theterminating acoustic loading in the horn results in a considerablyflatter frequency response than that obtainable with the horn alone.

Curves illustrative of typical measurements of harmonic distortionappear in Figure 5 wherein dashed line curve 32 illustrates the observeddistortion in an un terminated enclosure and curve 33 the observeddistortion in an acoustically loaded enclosure which has been loaded inaccordance with the practice of this invention. It is noted that thenon-uniform frequency response for an unloaded horn is particularlycritical in that the frequency response of the system is different atthe frequencies of the harmonics than it is at the fundamentalfrequency.

As a specific example of the practice of this invention a combinationacoustical horn and loud speaker assembly in the form of thatillustrated in Figures 1 and 2 may consist of an 8" loud speaker mountedin batfle member 18. The flare of the horn is approximately exponential.It is noted that a conical or hyperbolic flare may be utilized withoutdeparting from the spirit of this invention. If a hyperbolic flare isutilized it is possible to optimize the reactive components of the horn.The termination consists of approximately 5 pounds of fiber padding suchas that normally used for packing glass ware. Burlap, rags or othersound absorbing material may be used with equal success. The properamount of padding and its placement can be determined by impedanceand/or frequency response measurements since if the loading is notcorrect, the impedance characteristics will fluctuate. .The entireassembly is approximately 40 inches high with a sound radiating facearea of approximately 1.5 square feet.

This acoustical horn and speaker assembly provides a sound reproductiondown to a low frequency cutoff of the order of 40 cycles per second. Itis noted that a conventional labyrinth speaker and horn assembly couldnot give this response. A conventional non-loaded horn and speakerassembly incorporating an 8 inch speaker would normally have a volume ofabout 30 cubic feet for reasonably flat response and faithful frequencyreproduction. That is, the sound radiating face of such a conventionalunterminated horn speaker would have a sound radiating face area ofapproximately 16 square feet while a corresponding speaker and cabinetassembly such as that illustrated in Figures 1 and 2 has a volume ofabout 3.2 cubic feet with a sound radiating face area of approximately1.5 square feet.

It is noted that the horn assembly can be constructed out of anysatisfactory material such as pressed board, plywood or plastic. Thegiven dimensions are considered to be exemplary only, since the practiceof this invention can be carried out with any number of acousticaldiaphragm and horn assemblies depending on the particular frequencyresponse requirements and the available space.

Referring to Figure 6 of the drawing, which is a perspective view of analternative structure according to this invention, the assemblycomprises a cabinet 34 having a sound radiating face 35 across which agrill cloth 36 is normally positioned. Supported within the cabinet 34is an electro-acoustic transducer unit 37 includinga cone diaphragm 38which occupies a portion of the sound-radiating face 35 in a position todirect sound outward from the cabinet. A series of bafiles 39 on theinterior of the cabinet 34 define an approximately exponentiallyexpanding passageway positioned to direct sound waves from the back ofcone diaphragm 38 through the portion of the sound radiating face 35 notoccupied by the cone diaphragm 38.

The mouth of the exponential horn defined by the bafiles 39 has aterminating diaphragm 40 of soft padding material positioned across it.The material of the diaphragm 40 must be partially sound-absorbing andpartially sound-transmitting. For example, loosely felted wool or cottonfibers are satisfactory for this purpose as are a number of thicknessesof Wide mesh woven fabrics.

The presence of the terminating diaphragm 40 allows the cabinet 34 to besmaller than would be the case for an unterminated horn for the same lowfrequency cutoff. The mass of the terminating diaphragm 40 replaces themass of air in the mouth of an equivalent longer horn. Since the mouthend occupies the largest volume, this results in a volume saving asgreat as 50 percent. In addition, an impedance matching layer orstufling of sound absorbent material 41 is provided as an aid inobtaining the desired impedance match and frequency response.

While this invention has been described in connection with specificexemplary embodiments it is apparent that it is subject to numerousmodifications and it is intended in the appended claims to cover allmodifications and variations that come within the true spirit and scopethereof.

What I intend to claim by Letters Patent of the United States is:

1. An acoustic horn assembly comprising a cabinet having asound-radiating face, an acoustic diaphragm mounted in said cabinet withthe exterior side of said acoustic diaphragm coupled to a portion ofsaid sound radiating face, at least one baffle in said cabinet providinga horn for directing sound waves from the interior side of said acousticdiaphragm to a portion of said soundradiating face, and sound absorbingmaterial positioned in said horn providing a desired frequency responseand effectively terminating the horn with an impedance approximatelyequal to the characteristic impedance of the mouth of said horn.

2. An acoustic horn assembly comprising a cabinet having a soundradiating face, an acoustic diaphragm mounted in said cabinet and havingan exterior side coupled to said sound radiating face, at least onebathe in said cabinet providing a horn for directing the low frequencysound waves originating from the interior side of said acousticdiaphragm to the sound radiating face and acoustic loading meansincluding soft padding material positioned in the mouth of said horneffectively terminating the horn with an effective acoustical impedanceapproximately equal to the characteristic impedance of the mouth of saidhorn whereby reflections from the mouth of the horn are minimized.

3. An acoustic horn assembly comprising a cabinet having a soundradiating face, an acoustic diaphragm mounted in said cabinet with theexterior side of said acoustic diaphragm coupled to a portion of saidsound radiating face, at least one baffie in said cabinet providing ahorn for directing sound waves originating from the interior side ofsaid acoustic diaphragm to said portion of said sound radiating facecoupled to the exterior side of said acoustic diaphragm, and acousticimpedance means including soft padding material positioned in said hornto effectively terminate the horn with acoustical impedanceapproximately equal to the characteristic impedance of the mouth of saidhorn whereby reflections from the mouth of said horn are minimized.

4. An acoustic horn assembly comprising a cabinet having a soundradiating face, an electro-acoustic transducer unit including anacoustic diaphragm mounted in said cabinet with the exterior side ofsaid acoustic diaphragm coupled to a portion of said sound radiatingface, at least one baifle in said cabinet providing a horn for directingsound waves originating from the interior side of said acousticdiaphragm to a portion of said sound radiating face not coupled to theexterior side of said acoustic diaphragm, and a diaphragm positioned inthe mouth of said horn, said diaphragm having an acoustical impedanceapproximately equal to the characteristic impedance of the mouth of saidhorn.

5. An acoustic horn assembly of the type defined by claim 4 wherein saiddiaphragm positioned in the mouth of said horn includes sound absorbingmaterial.

6. An acoustic horn assembly comprising a cabinet having a soundradiating face, a cone type speaker diaphragm mounted in said cabinet,the exterior sound radiating surface of said cone being coupled to aportion of said sound radiating face, said cabinet providing a hornpassageway extending from the interier side of said cone and having itsmouth substantially flush with the portion of said sound radiating facenot occupied by said diaphragm and a terminating diaphragm includingsound absorbing material in the mouth of said horn, said diaphragmhaving an effective acoustic impedance approximate.y equal to thecharacteristic impedance of the mouth of said horn.

7. An acoustical diaphragm and horn assembly comprising an acousticaldiaphragm oriented to radiate high frequency components from the frontface thereof, a tapered acoustical horn having a throat portion coupledto the back face of said acoustical diaphragm and a mouth portionoriented in proximity to the front face of said acoustical diaphragm andacoustical loading means inserted in said horn to provide an effectiveacoustical impedance substantially equal to the characteristic impedanceof the mouth of said horn, said horn being dimensioned and loaded sothat the high frequency components are substantially attenuated toprovide a substantially fiat frequency response with a sharp cutoff sothat interference between radiation from the front and back faces of thediaphragm is minimized.

References Cited in the file of this patent UNITED STATES PATENTS929,482 Pearson July 27, 1909 1,373,943 Blandin Apr. 5, 1921 2,224,919Olson Dec. 17, 1940 2,293,181 Terman Aug. 18, 1942 2,604,182 Massa July22, 1952 FOREIGN PATENTS 929,692 France Jan. 5, 1948 641,718 GreatBritain Aug 16, 1950 143,597 Australia Sept. 27, 1951

